mac with comcast and two airport expresses

[ebarbash]

I have a two year old i mac and have used a RCA Lyra to route my music wirelessly to my two stereo systems. Lyra died and I bought two airport expresses. I am currently connected to the internet via Comcast and a wireless router and modum. I was finally able to get the Airport Expresses working (one AE connected to each receiver) BUT they show up on airport utility as three separate networks. I can either play one stereo system or another, or go onto the internet. I'd like to combine them so I can choose to any or all of them at once. I tried comcast and they had no way to tell me. Any ideas?